The field of this invention is inductor devices, and more particularly, broadband rf transformers for low impedance applications, especially where high power and/or low loss is also a consideration. A typical application is in output matching circuits for broadband rf transistor amplifiers above 30 watts.
The two-hole balun core as shown in FIG. 1 is commonly used for winding broadband rf transformers for use in impedance transformation, or isolation, or both. The primary concern is often to extend the bandwidth with minimum loss; and often power handling capacity, either pulse or continuous wave (cw), is also a concern. Low signal level applications of the two-hole balun core are described in Matsushima U.S. Pat. No. 3,449,704, and Duncan U.S. Pat. No. 4,052,785, for impedance transformations between 50.OMEGA. and 300.OMEGA..
The low frequency response is usually limited by the mutual inductive reactance, but often it may be limited by core losses or core saturation. In general, the low frequency pulse power capacity P.sub.p at a given frequency is proportional to the product of the frequency f, the core energy density U, and the core volume V. EQU P.sub.p =.alpha.f U V (1)
and EQU U.alpha.B.sub.s.sup.2 /.mu. (2)
where B.sub.s is the saturation flux density of the core and .mu. is the core permeability just prior to saturation. The proportionality factor in equation (1) is dependent on core geometry and winding geometry. This factor .alpha. is a maximum with closely spaced turns in a toroidal geometry. Broadhead U.S. Pat. No. 3,244,998 describes a method of using transmission line windings to achieve broadband response with a toroidal geometry for impedance transformations between 75.OMEGA. and 120.OMEGA.. This factor .alpha. is also nearly maximum for the D geometry described by Dacen U.S. Pat. No. 3,238,484, in which the flux path is similar to that in the present invention.
Typical nickel-zinc ferrites used in rf transformers have permeabilities ranging from 40 to 250 with typical saturation flux densities from 2000 to 2200 Gauss, while typical carbonyl iron powder materials have permeabilities ranging from 6 to 35 with typical saturation flux densities from 13,000 to 17,000 Gauss. Thus, energy densities of typical nickel-zinc ferrites range from 0.5 to 20.mu.J/cm.sup.3, while energy densities of typical carbonyl iron powder materials range from 4000 to 40,000.mu.J/cm.sup.3. Moreover, the iron powder materials generally have lower losses and better temperature stability. Yet the ferrites are much more widely used for broadband applications because their higher permeability allows higher mutual inductive reactance for a given length of wire, which is significant for broadband considerations.
The optimum conductor geometry is usually that which results in a transmission line impedance equal to the geometric mean of the input and output impedances. Such a condition is easy to achieve with impedances in the range of 20.OMEGA. to 200.OMEGA. using round wire and common two-hole balun cores, but it is very difficult to achieve at lower impedances using conventional techniques. Consequently, low impedance transformers generally use very small cores with a single turn for the low impedance winding. To simultaneously achieve adequate low frequency response, it has often been necessary to resort to high permeability ferrites with attendant high losses and low saturation flux densities and hence low power capacities. An alternate technique is to combine a number of higher impedance transformers in parallel, thus permitting higher power at low impedance.
The advantages of strip-lines in making low impedance transformers are discussed by Horn and Pitzalis, in U.S. Pat. No. 3,609,613 using a novel spiral winding geometry on a modified toroid core, for impedance transformations as low as 12.OMEGA. at frequencies up to 70 MHz. Holdeman U.S. Pat. No. 3,611,233, describes the use of twisted strip-lines on toroid cores for reduced reflections at 1000 MHz. Chesnel U.S. Pat. No. 4,079,324, describes a high power pulse transformer employing bucking strip-line windings to facilitate minimizing parasitic lead inductance for radar pulse modulation at frequencies up to 10 MHz and impedances as low as 1.OMEGA.; and Nyswander U.S. Pat. No. 4,092,621, describes the use of strip-line leads, integral with the windings, to reduce parasitic inductance for the same application.
The design guidelines for broadband, high power, low impedance rf transformers can be summed up as follows:
1. Select a core geometry which (A) maximizes inductance for a single turn coil of given length with a core material of given permeability, (B) simultaneously generates relatively uniform flux densities throughout the majority of the central core volume, (C) facilitates the use of low impedance transmission line conductors, and (D) gives a low thermal resistance from the regions of high flux density to the core surfaces.
2. Select a core material which (A) has a high energy density, (B) has a high permeability, (C) has low losses, (D) has low permittivity, and (E) has high thermal conductivity.
The common two-hole balun core scores well on criterion 1A of the above, but poorly on criteria 1B and 1C. The strip-line-core transformer of the present invention, while scoring somewhat lower than the two-hole balun on criteria 1A, scores much better on 1B, 1C, and 1D. Ferrites score better on 2B, but carbonyl iron powders score much better on 2A and 2C.